1. Field of the Invention
The invention relates to fabrication of silica bodies by colloidal sol-gel techniques.
2. Discussion of the Related Art
Optical transmission fiber typically contains a high-purity silica glass core optionally doped with a refractive index-raising element such as germanium, an inner cladding of high-purity silica glass optionally doped with a refractive index-lowering element such as fluorine, and an outer cladding of undoped silica glass. In some manufacturing processes, the preforms for making such fiber are fabricated by forming an overcladding tube for the outer cladding, and separately forming a rod containing the core material and inner cladding material. The core/inner cladding are fabricated by any of a variety of vapor deposition methods known to those skilled in the art, including vapor axial deposition (VAD), outside vapor deposition (OVD), and modified chemical vapor deposition (MCVD). MCVD is discussed in co-assigned U.S. Pat. Nos. 4,217,027; 4,262,035; and 4,909,816. MCVD involves passing a high-purity gas, e.g., a mixture of gases containing silicon and germanium, through the interior of a silica tube (known as the substrate tube) while heating the outside of the tube with a traversing oxy-hydrogen torch. In the heated area of the tube, a gas phase reaction occurs that deposits particles on the tube wall. This deposit, which forms ahead of the torch, is sintered as the torch passes over it. The process is repeated in successive passes until the requisite quantity of silica and/or germanium-doped silica is deposited. Once deposition is complete, the body is heated to collapse the substrate tube and obtain a consolidated core rod in which the substrate tube constitutes the outer portion of the inner cladding material. To obtain a finished preform, the overcladding tube is typically placed over the core rod, and the components are heated and collapsed into a solid, consolidated preform. It is possible to sinter a porous overcladding tube while collapsing it onto a core rod, as described in co-assigned U.S. Pat. No. 4,775,401.
Because the outer cladding of a fiber is distant from transmitted light, the overcladding glass generally does not have to meet the optical performance specifications to which the core and the inner cladding must conform. For this reason, efforts to both ease and speed manufacture of fiber preforms have focused on methods of making overcladding tubes. One area of such efforts is the use of a sol-gel casting process. Co-assigned U.S. Pat. No. 5,240,488 discloses a sol-gel process capable of producing crack-free overcladding preform tubes of a kilogram or larger. In this process, a colloidal silica dispersion, e.g., fumed silica, is obtained having a pH of 2 to 4. To obtain adequate stability of the dispersion and prevent agglomeration, the pH is raised to a value of about 10 to about 14 by use of a base. Typically, a commercially-obtained dispersion is pre-stabilized at such a pH value by addition of a base such as tetramethylammonium hydroxide (TMAH). Introduction of the TMAH raises the pH value. Other quaternary ammonium hydroxides behave similarly. When the pH is so raised, the silica takes on a negative surface charge due to ionization of silanol groups present on the surface, in accordance with the following reaction:
xe2x80x94Sixe2x80x94OH+OHxe2x88x92xe2x86x92xe2x80x94Sixe2x80x94Oxe2x88x92+H2O.
The negative charge of the silica particles creates mutual repulsion, preventing substantial agglomeration and maintaining the stability of the dispersion. In this state, the surface charge, and nominally the zeta potential, of the particles is at a negative value. (Zeta potential is the potential across the diffuse layer of ions surrounding a charged colloidal particle, and is typically measured from electrophoretic mobilitiesxe2x80x94the rate at which colloidal particles travel between charged electrodes placed in a solution. See, e.g., C. J. Brinker and G. W. Scherer, Sol-Gel Science, Academic Press, 242-243.)
At a later stage in the process, as discussed in Col. 15, lines 39-65 of the ""488 patent, a gelling agent such as methyl formate is added to reduce the pH. It is possible to use other esters, as well. The ester reacts to neutralize base, and the negative character of the silica particles is neutralized according to the following reaction:
xe2x80x94Sixe2x80x94Oxe2x88x92+H+xe2x86x92xe2x80x94Sixe2x80x94OH.
A sufficient amount of the ester must be introduced to neutralize the silica to a degree where gelation is induced. (Gelation, as used herein, indicates that the colloidal silica particles have formed a three-dimensional network with some interstitial liquid, such that the dispersion becomes essentially non-flowing, e.g., exhibiting solid-like behavior, at room temperature.) Subsequent to gelation, the sol-gel body is typically released from its mold, dried, heat treated, and sintered, as reflected in the Table at Cols. 11-12 of the ""488 patent.
As discussed in the ""488 patent, a major problem that had been encountered in sol-gel fabrication of relatively large bodies, e.g., 1 kg or greater, was cracking of the bodies during drying, heat treatment and/or sintering. In particular, the gel body undergoes substantial shrinkage from its gel form to its sintered form, e.g., typically greater than 10 linear percent shrinkage. This shrinkage induces numerous stresses in the body, and these stresses often lead to cracks. According to the ""488 patent, however, the inclusion in the sol of an extremely small amount of polymer additive, referred to as binder, reduced such cracking, particularly when a plasticizer was also used. (See, e.g., Col. 5, lines 19-56. ) For this reason, the formulation of the ""488 patent allowed fabrication of tubes of useful size in a commercially feasible manner.
However, the gel bodies produced by such sol-gel processes still undergo relatively substantial shrinkage from gel form to sintered form. This shrinkage continues to exert stresses throughout the body, and the gel bodies therefore require careful and highly controlled drying processes. These careful, controlled treatments are time-consuming and relatively costly. And the shrinkage reduces the number of uses for such sol-gel bodies. Processes which reduce the shrinkage and/or otherwise allow use of less time-consuming and costly techniques would be highly advantageous.
The invention provides a silica sol-gel fabrication process of the type described in the ""488 patent, but which allows improved control of the shrinkage that takes place during the drying of a gel body. Specifically, use of a particular polymeric additive makes it possible for a gel body to experience linear shrinkage, through the drying stage, at least 55% less than an identical process without the polymeric additive (meaning 100xc3x97(percent shrinkage without additivexe2x88x92percent shrinkage with additive)/percent shrinkage without additive). For example, it is possible to attain extremely low shrinkagexe2x80x94even below 1% linear shrinkage, in relatively large sol-gel bodies of (dry weight) 1 kg or more, typically 10 kg or more, and even 40 kg or more, by adding a sufficient amount and type of additive. (Percent linear shrinkage indicates 100xc3x97(initial lengthxe2x88x92final length)/initial length. The drying stage is complete when the body contains about 3 wt.% water or less.)
The shrinkage mechanism in silica sol-gel bodies has been modeled using classical drying theory, and this modeled mechanism is widely accepted. (See, e.g., C. J. Brinker and G. W. Scherer, supra, 453-509. ) Specifically, as drying occurs on the outer surface of a gel body, the solid material at the drying front is exposed to the ambient atmosphere. Because silica typically has surface silanols and thereby is hydrophilic, solid-vapor interfacial energy is greater than solid-liquid interfacial energy, i.e., the solid prefers to be wetted by the liquid rather than the vapor. To make this happen, liquid from the interior of the body flows toward the exterior to replace liquid that is evaporating from the outer surface. The flow of this liquid through a tortuous, fine pore matrix induces a pressure gradient, thereby putting the liquid in tension, and the resultant compressive force on the gel body causes shrinkage, as well as the tendency for crackingxe2x80x94a major obstacle in sol-gel manufacture. Thus, it is this hydrophilic property of silica that leads to shrinkage and cracking.
The process of the invention controls this shrinkage mechanism by providing in situ hydrophobicity to the silica during gelation. The additive generally contains a hydrocarbon chain having an attached hydrogen-acceptor moiety (e.g., an ether or carboxylic acid group), and is believed to function as follows. At the relatively high pH ( greater than 10) at which the colloidal silica sol is initially stabilized, a relatively low number of silanol groups are present on the surface of the silica particles due to de-protonation. Thus, adsorption of the polymer additive onto the silanols is also relatively low. However, as the gelling agent lowers the pH, surface silanol groups are re-formed, and thus an increasing amount of the additive adsorbs onto the silica particles. See, e.g., H. D. Bijsterbosch et al., xe2x80x9cNonselective Adsorption of Block Copolymers and the Effect of Block Incompatibility,xe2x80x9d Macromolecules, Vol. 31, 7436-44 (1998). This adsorption expectedly renders the silica surface increasingly hydrophobic (i.e., in situ hydrophobicity), because the oxygen-containing groups of the additive are oriented toward and hydrogen-bonded to the silanols, such that the exposed hydrocarbon chains form a hydrophobic coating. By making the silica hydrophobic, the propensity of the liquid to wet the silica of the gel body is substantially diminished, i.e., the liquid puts much less tension on the drying gel body. And the result is less shrinkage and substantially less cracking.
Surprisingly, one such polymeric additive is optionally the so-called binder referred to in the ""488 patent, but in higher concentrations than called for in the patent. It was unexpectedly discovered that providing an increased concentration of such binder, relative to the amounts called for in the ""488 patent, provided a significant decrease in shrinkage of gel bodies. For example, Col. 8, lines 37-55 of the ""488 patent instruct one to use, for a 40 to 50 wt. % silica dispersion, at most, 1 wt. % polymer based on the weight of the silica, but suggest that 0.5 wt. % or less is preferred. Higher amounts are indicated to lead to void formation and associated degradation of optical quality. Yet, it was discovered that higher amounts, for such silica loadings, are actually advantageousxe2x80x94in contrast to the teachings of the ""488 patent. The process of the present invention generally uses, for such loadings, greater than 1 wt. % of additive, more typically at least 3 wt. %, based on the weight of the silica, with the precise amount depending on, among other things, the particular additive, as discussed below.
By substantially lowering shrinkage, the invention makes it possible to increase the speed of drying, heat treatment, and sintering for sol-gel bodies. Moreover, the invention makes it possible to form more intricate and delicate bodies that are not feasible with current sol-gel techniques.